So, you're a bunch of sister-cells looking to get together and form the world's first animal co-op, a place where you and your buddies can all live together in a little socialist utopia and specialize in doing one chore, rather than trying to do everything at once like those foolish, single-celled, rugged-individualist protists. What might this look like?
Well, it just so happens it probably looked a lot like a sponge, because they, or something very like them, were likely the first. And this is what they look like today:
A sponge is, in essence, a multicellular organism with no organs or tissues, but with specialized cells, which distinguishes it from small multicellular protists. Like algae, the thinness of the sponge body plan and the fact that most of its cells are exposed to circulating water (provider of oxygen and food, remover of wastes) makes organs like hearts, kidneys, and digestive tracts unnecessary.
A typical barrel or tube sponge is in essence a perforated sac. Click here for a nice diagram that helps illustrate the following text. The exterior of sponges is coated with epidermal cells; in the wall of the sponge are wandering amoeboid cells that perform some immune-system like duties and secrete crystalline spicules (usually calcium carbonate or silicic acid) or proteinaceous fibers that support the sponge; special cells called porocytes can encircle the inflowing wall pores; and distinctive choanocytes, or collared cells, line the interior spaces of the sponge. The spicules can have any number of points; the fossils of the glass sponge Hydnoceras from the Devonian (390-360 mya) look for all the world like they're wrapped in plaid thanks to their six-pointed spicules which appear four-pointed in the two dimensions apparent on its exterior.
The collars that give choanocytes their name are made up of finger-like cytoplasmic membrane extensions called microvilli (like the ones that line your small intestine) surrounding tail-like flagella. The beating flagella create a current that draws water in through the porocytes or intercellular pores (in either case, the holes are called "ostia") and out through the central hollow of the sponge, called the osculum. Food particles that bump into the microvilli stick to them and are either engulfed and digested, or passed off to amoeboid cells who get the job done.
You can see the flow of water choanocytes generate most easily when sponges spawn. In this video, the divers capture sponges spawning at the Flower Garden Banks National Marine Sanctuary, in the Gulf of Mexico, some 70 to 115 miles off the coasts of Texas and Louisiana.
The sponges at the end of the video are undoubtedly releasing sperm. The first set of sponges may be females releasing eggs, but they are more likely hermaphrodites (most sponges are) and are releasing fertilized eggs or larvae. Sperm -- formed from a transmogrification of choanocytes -- must find another sponge of the proper species and fuse with a choanocyte inside. This is where it gets trippy. Once there, the sperm-bearing choanocyte metamorphoses into an amoeba and crawls toward an egg. The egg itself was formed when either a choanocyte or an amoebocyte engulfed special "nurse" cells to form a yolk. Sperm-bearing amoeboid choanocyte and engorged egg fuse. How the proper chromosomes find each other to form a single diploid nucleus in all this mess without ending up with five or six copies of the genome is a subject for another day.
The sponge can then release the fertilized eggs, or it can retain them. Either way, the zygote develops into a larva that consists of a ball of cells with external flagella or cilia drive the whole thing around. They tool about for a while praying they don't get eaten while looking for a suitable site to set up their choanoflagellate condo, and presto chango, the larva lands and grows into a sponge.
Sponges are often considered colonial organisms like the alga Volvox. The cells are so loosely associated that if you run a sponge through a filter and scramble the cells, they will re-associate easily to form another sponge. Slime molds, though evolved out of a completely different amoeboid lineage, can perform this trick too (Try *that* with any supposedly "higher" animal).
This brings us to the meat of this blog post, and the reason I include it in today's "Friends in Groups" Blog-a-thon. Ancestors of today's sponges may have been the the first truly multicellular animals. Over at S.E. Gould's Lab Rat blog is a nice definition of multicellularity, which says, essentially, cells of multicellular organisms must stick together, communcate with each other, depend on each other for survival, and specialize. Sponges seem to qualify by all these measures. To be fair, the placazoans, a fascinatingly simple roving cellular film, are also in the running for that honor, but I must save discussing them for another day.
How long ago this happened is a subject for debate, but it seems it was at least 543 million years ago during the Proterozoic from whence we know of the first fossil sponge spicules, and it could be much more. Early sponges may not have made spicules or any other traces likely to fossilize. Whenever they appeared, they must have cleaned up considerably what was probably becoming some pretty soupy seawater as they were and are, in essence, little ocean filtration units, the first of many filter feeders to come.
Regardless of which was the first multicellular animal, sponges and placozoans were probably not the first animals. That distinction may belong to protists called Choanoflagellates, which are single or multicellular bouquets or spheres of what look just like the choanocytes inside sponges. At least one free-living spherical choanoflagellate colony (coincidentally named Sphaeroeca volvox) looks not unlike a sponge larva.
In still other choanoflagellate species, the flagellate cells face *inward* into a cavity, much like, well, an adult sponge.
But, you may be saying, how do we know that such sedentary beings are animals at all, though? Well, their DNA sequences tells us nowadays. But biologists have known sponges are animals for much longer than we have known about DNA. Remember those proteinacious, elastic skeletons? (if you've ever squeezed a true bath sponge, you are familiar with this) Among other similarities, these fibers are made of a form of the protein collagen -- the very same protein in your tendons, ligaments, and skin, where, when it breaks down, you get wrinkles. And although no other animal groups have true choanocytes, most major groups (phyla) have "collar cells", and some of these may well have evolved from choanocyte ancestors. And your immune system also contains amoeboid "macrophages" that wander about your body at will, slipping in and out of tissues as they please, seeking invaders. No doubt there are other similarities.
Sponges were the first animal multicellular cooperatives, then. Some of these loose cellular federations, when subject to strong selection and other evolutionary forces, transformed into tightly knit multicellular cooperative machines like you: the rest of the animals. Their cells have become so bound up together that we no longer think of them as cellular co-ops. Animals are spoken of as one "organism". Yet when you shed skin cells or digestive cells, or when any cells in your body voluntarily die without offspring, they are sacrificing themselves for the good of this larger, albeit genetically identical, entity.
Before we leave the sponges, let's briefly look at some of the more entertaining types.
One of the three major groups of sponges are the glass sponges, distinguished by their glassy, six-pointed silicious spicules and sometimes-delicate, sculpted appearances. Here is one preserved in the Sant Ocean Hall at the Smithsonian. Notice its long rooting spicules, a feature many sponges share. These would be covered in mud in the ocean.
When sponges live deep -- really deep -- beyond the domain where filter feeding is a profitable business, they tend to become, amazingly, carnivorous. One might not expect such a feat for an organism that typically has all the mobility of a potted plant, but then again, plants have produced more than a few carnivores themselves.
One of the most bizarre-looking organisms on the planet is a carnivorous sponge. Don't believe me? Behold the ping-pong tree sponge, Chondrocladia lampoglobalis, a deep-sea dweller that resembles a 60's-retro chandelier popular among hipster home decorators. These curiosities are typically about 50 cm (20 in) high and live at 2600 to 3000 m (8,500 to 9800 ft) deep. Small crustaceans or worms so unfortunate as to land on the ping-pong tree sponge find themselves velcroed to it by tiny grappling-hook-like spicules. This triggers digestive cells to migrate to the site and spend the next few days chowing down.When supper is finished, they return to their original locations.
Other carnivorous sponges deploy similar weaponry. In addition to hooked spicules, they may use sticky threads to entangle and digest prey. Most carnivorous sponges have lost their choanocytes and resulting hydraulic systems, as these would be pointless in the absence of filter feeding. Chondrocladia has not. It uses a highly-modified hydraulic system to inflate the ping-pong balls at the ends of its branches.
A few sponges -- perhaps 50 species -- live in freshwater. My friend George Watson, a technical diver, snapped beautiful photos of this freshwater sponge deep -- probably beyond the reach of light -- in a small but *very deep* lake in, of all places, New Mexico. It is likely a species of Spongilla (and if anyone knows the exact species, George would like to know), the same genus as this specimen of Spongilla lacustris captured in the upper reaches of the Columbia River in Washington state:
You will notice that it is green. That is because it, like many freshwater sponges, hosts a green algal symbiont. Some marine species partner with blue-green algae (also called cyanobacteria) or even photosynthetic dinoflagellates (a kind of protist), which can provide over half of their energy demands in places where the filter feeding is not so hot. Ahh, biology. How I love you.
Finally, I cannot end this post without mentioning the plague of potato sponges that washed ashore following Hurricane Irene this year. The poor homely things seem to have been scoured from the seabed by the storm, whence they drifted, dying, ashore, only to be accused of being foul-smelling and disgusting. If your corpse were floating for several days in Chesapeake Bay, I'll wager it would not be so sweet-smelling either.